import time, threading
balance = 0
lock = threading.Lock()

def change_it(n):
    global balance
    balance = balance + n
    balance = balance - n

def run_thread(n):
    for i in range(10000):
        # 先获取锁
        lock.acquire()
        try:
            change_it(n)
        finally:
            # 改完要释放锁
            lock.release()

t1 = threading.Thread(target=run_thread, args=(5, ))
t2 = threading.Thread(target=run_thread, args=(8, ))
t1.start()
t2.start()
t1.join()
t2.join()
print(balance)